1. Field of the Invention
The present invention relates to architectures and designs of mitigation techniques for undesired acoustic noises. In particular, the present invention pertains to acoustic devices generating acoustic signal quiet zones for undesired noise mitigation, while simultaneously retaining desired acoustic signals at acceptable levels.
Several possible applications include, but are not limited to: 1) mitigation or elimination of road noise for in-car audio applications, 2) mitigation or elimination of engine noise for aeronautical applications, such as creation of individual quiet zones for passenger planes 3) mitigation of loud noises in enclosed areas such as cancellation of snoring in a bedroom, 4) creation mobile quiet zones, such as for music event type applications in relation to sound checking and engineering, 5) mitigation or elimination of noise signals in open areas, such as construction site equipment sound mitigation, 6) broad spectrum noise mitigation, such as for private housing near railroad tracks or airports.
2. Description of Related Art
Acoustic interference is a phenomenon that is so prevalent in our current society that it is notoriously difficult to avoid. Whether it be road noise being transmitted through a car frame to the car cabin, or noise generated by construction equipment, undesired noises permeate many facets of everyday life. By definition, acoustic interference, which are undesired acoustic signals (sound), originates from a source external to a desired signal path and produces undesired artifacts in the desired signal. The interfering signals in acoustic wave mode are referred as acoustic noises.
Acoustic interference may be the source of many problems associated with sound in our society today. Excessive amounts of noise can pose a health risk for the majority of the population, as hearing loss is a major concern. Excessive waste noise also causes problems, as it may devalue property (such as a house living near a railroad), make the area uncomfortable, among other things. Additionally, some noises may just be uncomfortable or undesirable, such as the snores of a partner, or the road noise within a car. Thus, there has been a need to find ways of mitigating such noise to improve many facets of our lives.
With our current technologies, the only viable solution to getting rid of these undesired noises without eliminating necessary acoustic signals (such as people's voices) has been to use noise cancelling technology. One of the best manifestations of this noise cancelling technology is Active Noise Control, or ANC. ANC involves using several microphones to generate a “noise signal,” which is then used to cancel out the undesired noises by playing the same signal back in inverted phases. There are three particular manifestations utilizing ANC which will be examined: noise cancelling headphones, ANC for automobiles, and audio feedback elimination.
Noise-Canceling Headphones
Noise cancellation headphones are one of the more obvious and transparent uses of this technology. These types of headphones make it possible for people to enjoy music without raising the volume excessively if there is too much acoustic interference. Noise-canceling headphones reduce unwanted ambient sounds (i.e., acoustic noise) by means of active noise control (ANC). This involves using one or more microphones placed near the ear, and electronic circuitry which uses the microphone signal to generate a “noise” signal with inverted phases. When the phase-inverted noise is produced by the speaker driver in the headphone, destructive interference cancels out the ambient noise as heard within the enclosed volume of the headphone.
Retail noise-cancelling headphones typically use ANC to cancel the lower-frequency portions of the noise; they depend on more traditional methods such as soundproofing to prevent higher-frequency noise from reaching the ear.
This approach is preferred because it reduces the demand for complicated electronic circuitry and at higher frequencies, where active cancellation is less effective. To truly cancel high frequency components (coming at the ear from all directions), the sensor and emitter for the canceling waveform would have to be adjacent to the user's eardrum, which is not technically feasible.
Noise-cancelling aviation headsets, typically circumaural headphones that enclose the wearer's ears completely, are also commonly available. This provides passive noise isolation so that electronic noise cancellation circuitry can perform better. Noise-canceling headphones have several limitations (that vary from vendor to vendor):                1. They work well for sounds that are continuous, such as the hum of a refrigerator or the sound in an airplane cabin, but are rather ineffective against speech or other rapidly changing audio signals.        2. They may introduce additional noise, usually in the form of high-frequency hiss.        3. They consume power, supplied by a USB port or a battery        
Bose® has continuously improved the technology since then. QuietComfort® 15 headphones reduce even more noise across the full spectrum of human hearing by advanced electronics with micro-phones both inside and outside each ear-cup to sense and reduce more of the sounds around a passenger. The advanced headphones feature less outside noise than ever before, and deliver better quality audio. This approach is preferred because it reduces the demand for complicated electronic circuitry and at higher frequencies, where active cancellation is less effective. However, like their previous incarnations, the headphones still have an issue with cancelling out high frequency, omnidirectional noise components. These headphones generally only cancel out constant, non-dynamic noise, and only up to 26 dB in power. The sensor and emitter for the canceling waveform have to be strategically and dynamically placed. Thus, these headphones still falter as they do not have a truly dynamic noise cancelling capability.
Active Noise Cancellation for Car
Honda is using noise-cancellation technology in their high end cars. The way it works is that a microphone connected to the car stereo system picks up all the sound inside the car, including music or such from the stereo. Then the noise-cancellation system subtracts the sound of the music coming from the stereo and produces noise-canceling sound waves that match the frequency of unwanted sound. The noise-canceling sound waves are also sent through the stereo speakers, along with the music. This technique greatly reduces the low frequency vibration noises in the car, without dampening the car's audio system. The system uses a microphone to hear what the driver and passengers hear, analyzes it with an onboard computer, and pipes out a cancelling sound via the stereo system.
Toyota in 2008 deployed a noise cancelling system for its Japanese-market Crown Hybrid that nearly eliminated engine sound within the passenger compartment. The system uses a complex system of microphones, speakers and sensors located around the cabin.
Like the headphones, the Toyota system works by using small microphones to monitor surrounding sounds, then plays back frequencies mathematically calculated to be the exact opposite of the ambient noise through the speakers, causing both sound waves to collide and cancel each other out. The Toyota system has an extra sensor that takes into account engine rpm.
Toyota has worked internal noise-canceling into its new Crown hybrid, with microphones to pick up engine and road noise, and then speakers to blast out anti-phase versions of those noises at head height. Toyota claims it can cut noise by around 5 to 8 dB.
Audio Feedback Elimination
Audio feedback is the ringing noise caused by a “looped signal”, that is, a signal which travels in a continuous loop; caused by interactions between microphones and speakers—a microphone feeds a signal into a sound system, which then amplifies and outputs the signal from a speaker, which is picked up again by the microphone. The feedback occurs when the gain in the signal loop reaches “unity” (0 dB gain).
There are many situations which result in feedback; such as the pickups of an electric guitar and speakers. To eliminate audio feedback, the feedback loop must be interrupted. There are many methods for controlling feedback, such as use of a more directional microphone, lowering the speaker output so that the microphone does not pick it up, speaking (or singing) close to the microphone, or positioning the microphone and/or speaker to minimize direct feeding of speaker output into the mic. However, these solutions are impractical. It either involves limiting the potential of the equipment, or physically changing the nature of the sound just to avoid feedback.
SDS had a patent filed for electromagnetic (EM) interference mitigation architectures in free space over a finite area, referred to as a quiet zone. The architectures consist of RF antenna geometries, and optimization processing. The RF antenna geometries must provide the following three functions: (1) directional and high fidelity pick-up of interference signals, (2) distributed emitters (array elements of an injection array), and (3) multiple sensors (probes in a quiet zone). Simulated results in FIG. 7 indicate the invented techniques work extremely well for directional interferences or localized strong RF jamming sources. The optimization processes are programmed for significant reduction of strong interference distributed over a selected quiet zone; an entire area, not for deep cancellations for a few spots. The mitigation architectures and techniques enable the wave propagation phenomena of continuous and effective destructive interference among multiple wavefronts over the quiet zone by precision injections of the interference signals themselves. The precision injections are through an injection array with distributed elements strategically located (physically or functionally equivalent) very close to the quiet zone.
This filing aims to take the principles of RF quiet zone generation and apply it to creating acoustic quiet zones. Audio quiet zone generation via the injection—for cancellation technique of this invention appears to be a viable and effective means to eliminate the audio feed back. The audio quiet zones are generated by precision controls on the amplitudes and phase weightings on the frequency components of signals radiated by audio monitors. The quiet zones will be continuously optimized and dynamically adjusted covering a volume where the beneficial users or heads of beneficial users are located.
The acoustic quiet zone generators must provide the following three functions for free space interfaces; (1) directional and high fidelity acoustic pick-up devices of interference signals (e.g. microphones), (2) distributed acoustic emitters (e.g. array of speakers or transducers), and (3) multiple sensors (e.g. acoustic probes or microphones) in a quiet zone.
Noise source pickups may be through in situ microphones or via microphone arrays. The embedded microphones will pick-up noise waveforms from localized individual noise sources or combinations of individual noise sources, while microphone arrays will have multiple acoustic beams focused individually to localized noise sources or directions of incoming interference signals. In situ microphones such as the recently invented MEM micro-phones may tie to various strategically identified locations at close proximity to a user on a structure or in air where acoustic noises propagating through. For example, to cancel selected engine noises from multiple engines on a commercial aircraft for a passenger but not voices or music from the public announcement systems, preferred locations for noise sampling shall be areas very closed to the passenger. Thus concurrent acoustic noise samples will be collected and processed for the selections of engine noises to be used in the cancellation loops.
The cancellation techniques generating quiet zones shall be applicable for acoustic waves. However, there are significant differences in processing; which must take the following into consideration:                1. Almost all acoustic waves in audible frequency range are longitudinal waves.        2. Required processing structures must be altered to account for relative percentage bandwidths.                    a. Information is in acoustic wave under direct modulation without carriers                        3. Dispersive natures on propagation especially multi-path effects                    a. Various frequency components via different propagation paths.                        